Reducing NOx emissions inside waste gas emissions generated in gas fuel water heaters is highly important with respect to environment and human health. In combustion systems, nitrogen oxides are formed in three different ways. These are as follows: formation of NOx which consists of nitrogen sources provided in the content of liquid or solid fuel, formation of NOx which are generated in the flame instantly but in small amounts and formation of thermal NOx at high temperatures.
Fuel-based NOx emission is generated as a result of reaction of the nitrogen included in the fuel content and the oxygen provided in the combustion air. No such problem is confronted in gas fuels. However, approximately half of total NOx emissions in solid and liquid fuels may be originated from nitrogen provided in the content of the fuel.
Formation of prompt NOx is constituted as a result of a fast reaction occurring between nitrogen present in the air and hydrocarbon radicals. Portion of these kind of NOx emissions inside total NOx emissions is quite low.
Thermal NOx formation occurs as a result of reaction of oxygen and nitrogen in combustion air at flame temperatures over particularly 1200° C. Thermal NOx emission increases very quickly as flame temperature increases. A great majority of NOx emissions released as a result of combustion of gas fuels occur in this way.
In the commercial natural gas water heating systems homogeneous combustion process is utilized as the combustion technology. High flame temperatures are reached under stoichiometric conditions during homogeneous combustion of natural gas. Thermal NOx formation takes place depending on high temperature under these conditions. The most efficient ways to reduce NOx emissions in combustion systems with gas fuels is to reduce the flame peak temperature and the residence time at these peak temperatures. Therefore, the systems used are mostly operated by excess air. In addition, secondary air supply can be provided to the combustion chamber in order to reduce flame peak temperature. Alternatively, formation of NOx can be prevented by reducing the flame temperature by absorbing thermal energy from flame by means of an appropriate apparatus at a suitable absorption rate.
Another method used in order to reduce NOx emissions is to start the homogeneous combustion process by rich fuel mixture and then to complete the combustion process by poor fuel mixture gradually. Combustion process is carried out consecutively in consecutive zones from rich mixture towards poor mixture in at least two zones. Gradual combustion is realized by injecting fuel or combustion air to consecutive combustion zones. The United State patent documents no. U.S. Pat. No. 5,195,884, U.S. Pat. No. 5,275,552, U.S. Pat. No. 7,198,482, U.S. Pat. No. 6,695,609 and the International patent document no. WO2010092150 can be cited as an example concerning this issue. In the said patent documents, homogeneous combustion systems wherein fuel supply nozzles are placed in different positions on the combustion chamber in order to realize the gradual combustion.
In homogeneous combustion technique, which is also called as diffusion flame, fuel and oxidant are mixed by means of diffusion and combustion reaction occurs in a combustion chamber wherein heat is also extracted from the system at the same time. Diffusion flame-type burners with reduced NOx emission are described in the patent documents no. U.S. Pat. No. 4,904,179 and EP1310737.
Another way to reduce NOx emissions released in combustion reaction is to reduce combustion temperature. Realizing combustion process at low temperatures is possible by only catalytic combustion. Catalytic combustion, which is also known as flameless combustion, occurs on a catalyst surface and with activation energies much lower than homogeneous combustion. In general, precious metal catalysts such as palladium and platinum are used. Chromium, manganese, iron, calcium, nickel, copper, zinc and tin oxides are also metals having oxidation capabilities and they can be used for the purpose of catalytic combustion. Due to the fact that methane, which is an intermediate compound of natural gas, is a highly symmetric molecule; it is required to be pre-heated to a temperature of approximately 250-400° C. in order to be burned catalytically. This pre-heating process affects the energy balance of combustion system in a negative way. In general; the attractiveness of palladium-based combustion catalyst is lost due to the fact that the PdO active sites of these catalysts are transformed into inactive metallic phase over 800° C.
The U.S. Pat. No. 5,464,006 and U.S. Pat. No. 5,810,577 disclose catalytic combustion systems in stages. In the U.S. Pat. No. 5,464,006, combustion takes place in two different catalytic combustion stages after the mixture of gas-fuel-air is passed through an electrical pre-heating zone. Approximately 70-90% of the fuel is burned in the first catalytic zone (catalytic gap burner tube) while the rest of the combustion takes place in the second catalytic zone and on a monolith-type catalyst. A similar application is also available in the U.S. Pat. No. 5,810,577.
The European Patent documents no. EP0256322 and EP0356709 disclose a heat exchange system which is immersed into a catalyst bed. Mixture of natural gas-air is heated to the temperature (320-390° C.) where catalytic combustion starts by means of an electrical heater or an electrical ignition system enabling homogeneous combustion in the beginning. After the catalytic combustion starts, combustion temperatures reach 400-700° C. and the said pre-heating systems are deactivated. Catalytic combustion reaction is over when the temperature decreases below 400 C. Pre-heating systems are temporarily re-activated for restart. Copper chromite is used as catalyst.
In the German Patent document no. DE3332572, combined surface and catalytic type burners are utilised in two consecutive combustion stages. In the first stage, primary combustion takes place on the combined (surface-catalytic) burner located in a position parallel to the second stage burner. The combustion system is completed, with a supplementary secondary air supply to the combustion gas leaving the first zone, by the identical second surface-catalytic combined burner system located in the lower part of the same reactor. The water fed to the heat exchanger units, which are located on the outlet of both burner pairs as connected in series, is heated by the heat of combustion gases.
The German Patent documents no. DE4308017, DE422711, DF4412714 and the European Patent document no. EP0671586 disclose systems having three combustion zones wherein surface-type burners and catalytic burners are used together. Combustion is carried out homogeneously by feeding part of the mixture of gas fuel-air into the surface-type burner. Whereas the gas fuel-air mixture, fed into the catalytic burner, is pre-heated over a heating jacket to temperatures of 300-350° C. by the heat generated in the surface-type burner. Thereby, the gap-type catalytic burner is activated. Lastly, the combined exhaust gases coming from both burners (surface-type burners, gap-type catalytic burner) enter the monolith-type catalytic burner and the combustion process is completed. The fuel having thermal energy of approximately 13 kW is burnt on a homogeneous surface-type burner while the remaining mixture of fuel-air is burnt catalytically by modulation in a thermal energy range of 6-12 kW. Hot water is obtained by providing water circulation in the chamber surrounding all three burners.
In the German Patent document no. DE19739704, two catalytic burners are used consecutively. The first catalytic burner unit is a ceramic block and it is also used as surface burner at the same time. On the inlet and outlet of the surface and catalytic burners, there are two heat exchanger units in series. The heat exchanger located in the burner inlet is designed so as to receive the heat emitted by radiation to prevent the flame to back fire. In addition, an amount of the heat composed is taken from the combustion chamber by the cooling water circulation wrapping the outside of the burning block.
In the European Patent document no. EP0789188, two catalytic burners are positioned consecutively in a similar way. There is one ignition electrode in the chamber between the two catalytic burners. Combustion process is initiated by the homogeneous combustion taking place on the first catalyst surface by means of the ignition electrode at first. In order to prevent the catalysts from being overheated, cooling plates with IR-radiation absorption layers are placed on both sides of the chamber where the ignition electrode is located. Combustion is completed by burning the fuel fraction, which is not burnt in the first catalytic burner, in the second catalytic burner with monolith geometry. The ignition electrode, which is described in the said patent document used for first ignition, can be positioned in the zone remaining in between the two catalytic burners and it can also be positioned in the zone remaining in between the cooling-distribution plate put to the side of gas supply and the first catalytic combustion plate. Alternatively in the patent document it is described that it is possible to place the two units of the system, which consists of the ignition electrode positioned in the zone in between the two catalytic burners and the catalytic burners, parallel to each other.
In the German Patent document no. DE4324012, homogeneous combustion and catalytic combustion are carried out successively. Exhaust gases and unburned hydrocarbons occurring as a result of homogeneous combustion are passed over a catalytic type burner plate and thereby exhaust gases with reduced emission are taken out from the unit to the exhaust pipe. The actual combustion occurs in the homogeneous burner. The catalytic burner is used with the purpose of oxidizing volatile organic compounds to provide an improvement in exhaust gas emissions. In this system proposed, there is no heat exchanger for hot water production.
The U.S. Pat. No. 5,851,489 discloses a diffusion-type catalytic combustion system. Fuel is diffused into the support structure, where the catalyst is impregnated, from the inner part while air is diffused from the outer surface on the same structure. Catalytic combustion occurs on the catalyst support structure and temperatures reach 400-750 C. A liquid (for example: water) can be heated by means of a heat jacket placed to the section remaining in between the surfaces.
In the U.S. Pat. No. 6,431,856, the pre-mixed mixture of fuel-air is fed into the combustion chamber. Homogeneous combustion is initiated by means of an ignition electrode located in the entry of the combustion chamber and the catalyst block is pre-heated to a desired temperature. After the catalyst block is heated to the temperatures where catalytic combustion will start, the mixture of fuel-air is interrupted and it is ensured that the flame is extinguished. Catalytic combustion starts on the hot catalytic surface by re-feeding the mixture of fuel-air one after the other while the ignition electrode is deactivated. Whereas the water, which is circulated from the heat exchanger, located behind the catalytic burner and in the exhaust line, is heated by means of exhaust gases.
The U.S. Pat. No. 7,444,820 discloses a two-stage catalytic combustion process for gas turbines. Catalytic combustion is carried out by the rich mixture from the first catalytic combustion unit. Temperature of the hot air exiting the compressor is sufficient in order to reach the temperatures where catalytic combustion starts by rich mixture. As a result of the combustion occurring in the first catalytic burner by rich mixture, hot gas fuel (with H2,CO content) comprising flammable hydrocarbon components occur due to the fact that complete combustion does not happen. Part of the heat, which occurs as a result of rich combustion, is transferred over the heat exchanger to the combustion air and the secondary combustion air is heated for poor combustion. Partially oxidized hydrocarbons are mixed with the secondary combustion air, such that a poor mixture will be formed and complete combustion is carried out in the secondary catalytic combustion unit.
In the U.S. Pat. No. 5,052,919, a two-stage homogeneous combustion is carried out. During the coal gasification process a gas with high ammonia content occurs in the coal gasification process. A high amount of NOx emission occurs as a result of burning this ammonia-containing gas under stoichiometric conditions. In order to reduce the NOx emissions, a two-stage homogeneous combustion is described in the said patent. An important part of the fuel is burned under rich combustion conditions at lambda values of 0.6 to 0.9.